1. Field of the Invention
This invention relates to electric arc heater apparatus and method and, more particularly, it pertains to such apparatus for the production of acetylene from heavy hydrocarbons such as coal and liquid or gaseous petroleum feedstock.
2. Description of the Prior Art
Acetylene produced from coal is becoming economically more attractive as the prices of light hydrocarbon feedstocks increase. A principal use for acetylene would be in the production of vinyl chloride which is presently produced from ethylene. Acetylene can be substituted for ethylene for a large number of additional processes. From the technical studies performed by several independent sources on the production of acetylene from coal, it appears that this process is close to being economically competitive with ethylene in the production of several intermediate products.
In one study the arc heater used was powered with a water cooled metal anode and either a carbon or water cooled metal cathode. The arc was rotated by means of an axial magnetic field, the source of which was a DC coil around the tubular shaped arc heater. The coal fed into the arc heater dropped through this rotating arc that might be idealized as a spoke in a wheel as it rotates around the center electrode. As the coal and gas go through the arc, they rapidly reach temperatures of from about 2000.degree. to well above 4000.degree. K. At these temperatures acetylene is the most prevalent of the pyrolysis products. If the high temperature acetylene were cooled slowly to the temperature range of 600.degree. to 1200.degree. K, it would readily decompose into less valuable products. So it is necessary for the high temperature gaseous products in the arc to be rapidly quenched to temperatures below 600.degree. K to avoid any decomposition of the acetylene.
To quench the exit gas, pulverized coal, hydrogen, or recycled hydrocarbon liquid or gas from the process is introduced into the stream and mixed with it thoroughly, thus dropping the temperature to below 600.degree. K. This rapid quenching minimizes the decomposition of the acetylene. After quenching, the gases are processed to separate the different valuable components such as char, carbon black, ethylene, cyanide, and other chemicals all of which are commercially desirable products. By using a rotating radial DC arc, the principal problem was the formation of deposits on the arc heater wall. These deposits consisted of a hard carbon crust that was not easily removed. To prevent the formation of the larger crust very frequent washes with steam or water were required to remove these deposits in their finely divided state, before they could form a larger crust. In their operation they were required to shut down power and the coal and gas feeds every minute for a three second wash. The overall effect on production time in this mode of operation would be significantly greater than the three seconds stated due to lags in startup and shutdown. This washing operation not only caused a lower productivity for a given power arc heater but it also meant that there was an additional capital expense for power and control equipment, and higher operating costs due to increased utilities as well as maintenance.